Regarding a switching power supply apparatus in which input and output sides thereof are electrically isolated by a transformer, PSR (primary-side regulated type) switching power supply apparatus has been known in which an auxiliary winding, to which a voltage proportional to a voltage generated to a secondary winding of the transformer is induced, is provided on a primary side to thereby regulate an output voltage based on the voltage induced in the auxiliary winding (e.g. U.S. Pat. No. 8,125,799 B2 and US 2012/0134182 A1).
The switching power supply apparatus described in the Patent Documents has a circuit which detects reduction in an output voltage resulting from a sudden load change from light load or no load and thus outputs a voltage reduction signal to the secondary winding. The switching power supply apparatus performs a switching operation based on the voltage reduction signal transferred from the secondary winding to the auxiliary winding to thereby suppress the reduction in the output voltage resulting from the sudden load change.